Sept 8 (Reuters) - Hurricane Irma will pose the toughest test yet for U.S. nuclear power plants since reactors strengthened their defenses against natural disasters following the Fukushima Daiichi nuclear accident in Japan in 2011.

Irma was on course to hit South Florida early on Sunday as a Category 4 storm, packing winds of up to 145 miles per hour and bringing a storm surge of as much as 12 feet to a state that is home to four coastal nuclear reactors.

The National Hurricane Center's forecast track shows Irma making landfall on the southwest side of the Florida Peninsula, west of the two nuclear reactors at the Turkey Point plant.

The operator, Florida Power & Light (FPL), has said it will shut Turkey Point well before hurricane-strength winds reach the plant. The reactors are about 30 miles south of Miami.

28PHOTOS

Preparing for Hurricane Irma

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Preparing for Hurricane Irma

YEMASSEE, SC - SEPTEMBER 08: Northbound lanes of I-95 near the Georgia-South Carolina border are empty as northbound lanes are packed as pepole evacuate ahead of the arrival of Hurricane Irma September 8, 2017 in Yemassee, South Carolina. Florida appears to be in the path of the hurricane which may come ashore at category 4. (Photo by Chip Somodevilla/Getty Images)

ST. PETERSBURG, FL - SEPTEMBER 05: Stan Glass, of St. Petersburg, fills four 5-gallon fuel tanks with gasoline for his boat should he have to evacuate by boat as residents in the area prepare ahead of Hurricane Irma on September 05, 2017 in St. Petersburg, Florida. The National Hurricane Center (NHC) has reported that Hurricane Irma has strengthened to a Category 5 storm as it crosses into the Caribbean and is expected to move on towards Florida. (Photo by Brian Blanco/Getty Images)

A woman looks at empty shelves that are normally filled with bottles of water after Puerto Rico Governor Ricardo Rossello declared a state of emergency in preparation for Hurricane Irma, in San Juan, Puerto Rico September 4, 2017. REUTERS/Alvin Baez

People buy materials at a hardware store after Puerto Rico Governor Ricardo Rossello declared a state of emergency in preparation for Hurricane Irma, in Bayamon, Puerto Rico September 4, 2017. REUTERS/Alvin Baez

As Hurrcane Irma approaches Florida September 8, 2017 shoppers in Port St. John, near Kennedy Space Center, find almost empty shelves.
Warning that Irma would be worse than Hurricane Andrew, which killed 65 people in 1992, Florida's governor said all of the state's 20.6 million inhabitants should be prepared to evacuate. / AFP PHOTO / BRUCE WEAVER (Photo credit should read BRUCE WEAVER/AFP/Getty Images)

People buy materials at a hardware store after Puerto Rico Governor Ricardo Rossello declared a state of emergency in preparation for Hurricane Irma, in Bayamon, Puerto Rico September 4, 2017. REUTERS/Alvin Baez

Workers put boats on dry docks in preparation, as Hurricane Irma, barreling towards the Caribbean and the southern United States, was upgraded to a Category 4 storm, in San Juan, Puerto Rico September 4, 2017. REUTERS/Alvin Baez

Customers walk near empty shelves that are normally filled with bottles of water after Puerto Rico Governor Ricardo Rossello declared a state of emergency in preparation for Hurricane Irma, in San Juan, Puerto Rico September 4, 2017. REUTERS/Alvin Baez

People buy materials at a hardware store after Puerto Rico Governor Ricardo Rossello declared a state of emergency in preparation for Hurricane Irma, in Bayamon, Puerto Rico September 4, 2017. REUTERS/Alvin Baez

People buy material at a hardware store after Puerto Rico Governor Ricardo Rossello declared a state of emergency in preparation for Hurricane Irma, in Bayamon, Puerto Rico September 4, 2017. REUTERS/Alvin Baez

Workers put boats on dry docks in preparation, as Hurricane Irma, barreling towards the Caribbean and the southern United States, was upgraded to a Category 4 storm, in San Juan, Puerto Rico September 4, 2017. REUTERS/Alvin Baez

Hurricane Irma, a record Category 5 storm, churns across the Atlantic Ocean on a collision course with Puerto Rico and the Virgin Islands, is shown in this NASA GOES satellite image taken at 1715 EDT (2215 GMT) on September 5, 2017. Courtesy NASA/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY

Members of the Civil Defense prepare their gear ahead of Hurricane Irma, in Santo Domingo, Dominican Republic September 5, 2017. REUTERS/Ricardo Rojas

A member of the Emergency Operations Committee (COE) monitors the trajectory of Hurricane Irma in Santo Domingo, Dominican Republic September 5, 2017. REUTERS/Ricardo Rojas

A member of the Emergency Operations Committee (COE) monitors the trajectory of Hurricane Irma in Santo Domingo, Dominican Republic September 5, 2017. REUTERS/Ricardo Rojas

Shoppers in a Home Depot store wait for plywood in the Little Havana neighborhood in Miami, Florida, September 5, 2017. Residents are preparing for the approach of Hurricane Irma. REUTERS/Joe Skipper

Men cover the windows of a auto parts store in preparation for Hurricane Irma, in San Juan, Puerto Rico September 5, 2017. REUTERS/Alvin Baez

A man uses a cable to secure the roof of his home in preparation for Hurricane Irma, in Toa Baja, Puerto Rico September 5, 2017. REUTERS/Alvin Baez

Men cover the window of a house in preparation for Hurricane Irma, in Toa Baja, Puerto Rico September 5, 2017. REUTERS/Alvin Baez

Empty boxes of produce at Costco as customers purchased all the product on September 5, 2017 in Miami.
The monster hurricane coming on the heels of Harvey, which struck Texas and Louisiana late last month, is expected to hit a string of Caribbean islands including Guadeloupe late Tuesday before heading to Haiti and Florida. The Miami-based National Hurricane Center said Irma had strengthened to the most powerful Category Five, packing winds of 180 miles (280 kilometers) per hour.
/ AFP PHOTO / Michele Eve Sandberg (Photo credit should read MICHELE EVE SANDBERG/AFP/Getty Images)

Shoppers at Costco buying essentials in preparation for Hurricane Irma on September 5, 2017 in North Miami.
The monster hurricane coming on the heels of Harvey, which struck Texas and Louisiana late last month, is expected to hit a string of Caribbean islands including Guadeloupe late Tuesday before heading to Haiti and Florida. The Miami-based National Hurricane Center said Irma had strengthened to the most powerful Category Five, packing winds of 180 miles (280 kilometers) per hour.
/ AFP PHOTO / Michele Eve Sandberg (Photo credit should read MICHELE EVE SANDBERG/AFP/Getty Images)

Costco ran out of water as people shop to prepare for Hurricane Irma on September 5, 2017 in North Miami.
The monster hurricane coming on the heels of Harvey, which struck Texas and Louisiana late last month, is expected to hit a string of Caribbean islands including Guadeloupe late Tuesday before heading to Haiti and Florida. The Miami-based National Hurricane Center said Irma had strengthened to the most powerful Category Five, packing winds of 180 miles (280 kilometers) per hour.
/ AFP PHOTO / Michele Eve Sandberg (Photo credit should read MICHELE EVE SANDBERG/AFP/Getty Images)

Very long checkout lines at Costco as some people waited up to 8 hours to check in, shop and leave in preparation for Hurricane Irma on September 5, 2017 in North Miami.
The monster hurricane coming on the heels of Harvey, which struck Texas and Louisiana late last month, is expected to hit a string of Caribbean islands including Guadeloupe late Tuesday before heading to Haiti and Florida. The Miami-based National Hurricane Center said Irma had strengthened to the most powerful Category Five, packing winds of 180 miles (280 kilometers) per hour.
/ AFP PHOTO / Michele Eve Sandberg (Photo credit should read MICHELE EVE SANDBERG/AFP/Getty Images)

A woman takes a photo of a boarded up business in advance of Hurricane Irma's expected arrival in Fort Lauderdale, Florida, U.S., September 8, 2017. REUTERS/Carlo Allegri

YEMASSEE, SC - SEPTEMBER 08: Northbound lanes of I-95 near the Georgia-South Carolina border are empty as northbound lanes are packed as pepole evacuate ahead of the arrival of Hurricane Irma September 8, 2017 in Yemassee, South Carolina. Florida appears to be in the path of the hurricane which may come ashore at category 4. (Photo by Chip Somodevilla/Getty Images)

As Hurrcane Irma approaches Florida September 8, 2017 residents of Titusville, near Kennedy Space Center, have arleady exhausted the lumber yards of plywood used to board up windows. Irma is expected to arrive in the area Sunday afternoon, September 10th.
Warning that Irma would be worse than Hurricane Andrew, which killed 65 people in 1992, Florida's governor said all of the state's 20.6 million inhabitants should be prepared to evacuate. / AFP PHOTO / BRUCE WEAVER (Photo credit should read BRUCE WEAVER/AFP/Getty Images)

As Hurrcane Irma approaches Florida September 8, 2017 residents of Titusville, near Kennedy Space Center, have stop for last minute items and fuel. Gas prices had already been raised because of Hurricane Harvey hitting Texas, making gasoline cost more per gallon than diesel fuel.
Warning that Irma would be worse than Hurricane Andrew, which killed 65 people in 1992, Florida's governor said all of the state's 20.6 million inhabitants should be prepared to evacuate. / AFP PHOTO / BRUCE WEAVER (Photo credit should read BRUCE WEAVER/AFP/Getty Images)

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FPL said it will also shut the other nuclear plant in Florida at St Lucie, which also has two reactors on a barrier island on the state's east coast, about 120 miles north of Miami.

FPL said both Turkey Point and St Lucie were designed to withstand storms stronger than any ever recorded in the region and both plants are elevated 20 feet above sea level to protect against flooding and extreme storm surges.

But South Miami Mayor Philip Stoddard said he was concerned about the potential for floods to damage power generators at Turkey Point, which in turn might threaten the ability of the plant to keep spent nuclear fuel rods cool. At Fukushima in Japan, an earthquake and tsunami disrupted power supplies and caused the fuel in some units to meltdown.

"The whole site is pretty well able to handle dangerous wind, the real problem from my perspective is water," Stoddard said. He said he was more worried about the nuclear waste than the reactors.

"The real question is can they keep the spent fuel cool."

Peter Robbins, an FPL spokesman, said “Mayor Stoddard is wrong. Turkey Point is safe and is ready for Hurricane Irma.”

U.S. nuclear operators have taken steps to improve preparations for disasters since Fukushima. The U.S. Nuclear Regulatory Commission (NRC) required plants to install portable pumps and generators to keep water moving over fuel rods and the spent fuel pool even if offsite power supply was lost.

Nuclear plants also hired more staff and stored equipment needed to deal with reactor problems.

"Things are better today than in March 2011. Time will tell whether better proves good enough," said Dave Lochbaum, director of a watchdog group, the nuclear safety project, at the Union of Concerned Scientists.

The NRC said it was posting more inspectors at the two Florida plants and was considering sending more inspectors to plants in Georgia and South Carolina should the storm head that way.

The NRC said the plants should not be compromised by Irma's storm surge.

"The storm surge forecasts that we have seen so far do not challenge the sites' designs," said NRC spokesman Scott Burnell.

16PHOTOS

Hurricane Irma's wrath from above

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Hurricane Irma's wrath from above

Hurricane Irma (L) and Hurricane Jose are pictured in the Atlantic Ocean in this September 7, 2017 NOAA satellite handout photo. NOAA/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY.

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma on Maho beach, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma, rampaging across the Caribbean, has produced sustained winds at 295 kilometres per hour (183 miles per hour) for more than 33 hours, making it the longest-lasting, top-intensity cyclone ever recorded, France's weather service said on September 7. / AFP PHOTO / DUTCH DEFENSE MINISTRY / GERBEN VAN ES / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

CARIBBEAN SEA - SEPTEMBER 7: In this NOAA handout image, NOAA's GOES satellite shows Hurricane Irma as it moves towards the Florida Coast in the Caribbean Sea taken at 20:00 UTC on September 07, 2017. The state of Florida is in the track of where the hurricane may make landfall. (Photo by NOAA GOES Project via Getty Images)

TOPSHOT - An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma, rampaging across the Caribbean, has produced sustained winds at 295 kilometres per hour (183 miles per hour) for more than 33 hours, making it the longest-lasting, top-intensity cyclone ever recorded, France's weather service said on September 7. / AFP PHOTO / DUTCH DEFENSE MINISTRY AND AFP PHOTO / GERBEN VAN ES / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma, rampaging across the Caribbean, has produced sustained winds at 295 kilometres per hour (183 miles per hour) for more than 33 hours, making it the longest-lasting, top-intensity cyclone ever recorded, France's weather service said on September 7. / AFP PHOTO / DUTCH DEFENSE MINISTRY AND AFP PHOTO / GERBEN VAN ES / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

CARIBBEAN SEA - SEPTEMBER 7: In this NOAA handout image, NOAA's GOES satellite shows Hurricane Irma as it moves towards the Florida Coast in the Caribbean Sea taken at 16:15 UTC on September 07, 2017. Irma is a category 5 hurricane and will bring life-threatening wind, storm surge, and rainfall hazards to the Virgin Islands and Puerto Rico. The state of Florida is in the track of where the hurricane may make landfall. (Photo by NOAA GOES Project via Getty Images)

CARIBBEAN - AUGUST 25: In this NOAA handout image, NOAA's GOES satellite shows Hurricane Irma as it makes its way across the Atlantic Ocean in to the Caribbean -- a category 5 storm with winds as high as 185 miles per hour -- today at about 3:15 pm (eastern), September 6, 2017. (Photo by NASA/NOAA GOES Project via Getty Images)

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma, rampaging across the Caribbean, has produced sustained winds at 295 kilometres per hour (183 miles per hour) for more than 33 hours, making it the longest-lasting, top-intensity cyclone ever recorded, France's weather service said on September 7. / AFP PHOTO / DUTCH DEFENSE MINISTRY / GERBEN VAN ES / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma, rampaging across the Caribbean, has produced sustained winds at 295 kilometres per hour (183 miles per hour) for more than 33 hours, making it the longest-lasting, top-intensity cyclone ever recorded, France's weather service said on September 7. / AFP PHOTO / DUTCH DEFENSE MINISTRY / GERBEN VAN ES / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma, on the Princess Juliana International Airport and Simpson Bay Beach, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma, rampaging across the Caribbean, has produced sustained winds at 295 kilometres per hour (183 miles per hour) for more than 33 hours, making it the longest-lasting, top-intensity cyclone ever recorded, France's weather service said on September 7. / AFP PHOTO / DUTCH DEFENSE MINISTRY / GERBEN VAN ES / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS
/ The erroneous mention appearing in the metadata of this photo by GERBEN VAN ES has been modified in AFP systems in the following manner: [SIMPSON BAY BEACH] instead of [MAHO BEACH. Please immediately remove the erroneous mention from all your online services and delete it from your servers. If you have been authorized by AFP to distribute it to third parties, please ensure that the same actions are carried out by them. Failure to promptly comply with these instructions will entail liability on your part for any continued or post notification usage. Therefore we thank you very much for all your attention and prompt action. We are sorry for the inconvenience this notification may cause and remain at your disposal for any further information you may require. (Photo credit should read GERBEN VAN ES/AFP/Getty Images)

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma in Philipsburg, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma sowed a trail of deadly devastation through the Caribbean on Wednesday, reducing to rubble the tropical islands of Barbuda and St Martin. / AFP PHOTO / ANP / Gerben van Es / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - NO ARCHIVES - NO SALE- DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma, rampaging across the Caribbean, has produced sustained winds at 295 kilometres per hour (183 miles per hour) for more than 33 hours, making it the longest-lasting, top-intensity cyclone ever recorded, France's weather service said on September 7. / AFP PHOTO / DUTCH DEFENSE MINISTRY / GERBEN VAN ES / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma in Philipsburg, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma sowed a trail of deadly devastation through the Caribbean on Wednesday, reducing to rubble the tropical islands of Barbuda and St Martin. / AFP PHOTO / ANP / Gerben van Es / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - NO ARCHIVES - NO SALE- DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

TOPSHOT - An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma in Philipsburg, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma sowed a trail of deadly devastation through the Caribbean on Wednesday, reducing to rubble the tropical islands of Barbuda and St Martin. / AFP PHOTO / ANP / Gerben van Es / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - NO ARCHIVES - NO SALE- DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

An aerial photography taken and released by the Dutch department of Defense on September 6, 2017 shows the damage of Hurricane Irma in Philipsburg, on the Dutch Caribbean island of Sint Maarten.
Hurricane Irma sowed a trail of deadly devastation through the Caribbean on Wednesday, reducing to rubble the tropical islands of Barbuda and St Martin. / AFP PHOTO / ANP / Gerben VAN ES / Netherlands OUT / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT 'AFP PHOTO / DUTCH DEFENSE MINISTRY/GERBEN VAN ES' - NO MARKETING NO ADVERTISING CAMPAIGNS - NO ARCHIVES - NO SALE- DISTRIBUTED AS A SERVICE TO CLIENTS
(Photo credit should read GERBEN VAN ES/AFP/Getty Images)

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Both Florida reactors have previously weathered major storms. Turkey Point took a hit from Hurricane Andrew, a Category 5 storm, in 1992. Andrew damaged a smokestack at the plant. Repairs cost about $90 million.

FPL shut St Lucie last October as Hurricane Matthew skirted the Florida coast.

Lochbaum said the NRC and the industry could do more to reduce vulnerability to flooding.

In January 2014, about 50,000 gallons of rainwater leaked into the St Lucie plant after a heavy downpour. An NRC study blamed degraded and missing flood seals that were not discovered during checks after Fukushima.

There is also spent nuclear fuel at the site of a third power plant in Florida which stopped operating in 2009. That is the Crystal River plant, owned by Duke Energy Corp.

Duke is in the process of transferring used fuel from the spent fuel pool at the plant to dry cask storage as part of work to decommission the plant. Once in storage, the fuel no longer needs cooling.

The NRC's Burnell said Duke has suspended work to transfer the rods ahead of Irma. He said the fuel was safe and the plant also has backup power, even though it has been shut for years.

Irma is expected to disrupt much of Florida's power supply. FPL, the state's largest electric company, has warned Irma could cut service to about 4.1 million of its nearly 5 million customers. FPL is one of four large publicly traded utilities in Florida.

Other natural disasters since Fukushima have shut some plants. An earthquake in Virginia in 2011 shut Dominion Energy Inc's North Anna plant for about 2-1/2 months, the time it took to complete a full damage inspection. That plant sustained no major damage.

Hurricane Sandy in 2012 caused three reactors in the U.S. Northeast to shut but inflicted no serious damage.